Timstone: Actually if I recall correctly, the First Ones even before the Vorlons and the Shadows build some jumpgates to stimulate THEIR growth. Those where not required later on as both races learned to open jumpgates "on the fly". However this is so long ago that none of these can be found. (interesting including them very very rarely tho)
Anyways to the point: there are actually malfunctioning/damaged jumpgates tho i can't recall an example out of my bare head.
As for destroying of Jumpgates, if I recall correctly the first time Sheridan takes the Whitestar out they run into a Shadow Vessel which they can't handle and they blow up a Jumpgate by opening a jumppoint inside a jumpgate
I am bad with rembering names/episodes so I could be wrong here but I believe it's episode "Matters of Honor." Season Three, Episode 1.